Optical coupling apparatus and methods of making same

This application is a continuation of U.S. patent application Ser. No. 18/081,525, filed Dec. 14, 2022, now U.S. Pat. No. 11,808,998, issued Nov. 7, 2023, which is a division of U.S. patent application Ser. No. 17/186,661, filed Feb. 26, 2021, now U.S. Pat. No. 11,531,173, issued Dec. 20, 2022, the contents of each is incorporated by reference herein in their entireties

Optical gratings are frequently used to couple light between a waveguide and an optical fiber. Due to extremely different dimensions of the waveguide and the optical fiber, direct coupling would incur tremendous light loss. It is thus essential to meticulously design a waveguide light coupling apparatus for light mode field matching to the fiber dimension.

For example, an incoming light to a waveguide is usually in an unknown and arbitrary polarization state, such that a polarization splitting grating coupler (PSGC) is needed to provide polarization light in either transverse magnetic (TM) or transverse magnetic (TE) polarization mode from the optical fiber to the waveguide. The coupling efficiency of a PSGC is typically impacted by a polarization dependent loss (PDL) of TE and TM modes, which may result from non-zero fiber angle used to minimize reflections at the interface between fiber and grating. A conventional PSGC includes circular or square scattering elements at the intersection of grating lines on the grating, which results in high polarization dependent loss between TE and TM modes and degrades coupling efficiency.

As such, there exists a need to develop a method and apparatus of efficient optical coupling using optical gratings.

The following disclosure describes various exemplary embodiments for implementing different features of the subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, it will be understood that when an element is referred to as being “connected to” or “coupled to” another element, it may be directly connected to or coupled to the other element, or one or more intervening elements may be present.

A waveguide surrounded by a cladding layer may confine light based on refractive index contrast between the materials in the waveguide and the cladding layer. For example, a silicon waveguide with sub-micron dimension can confine infrared light (with a wavelength larger than about 700 nanometers or 700 nm) due to its strong refractive index contrast to its silicon oxide cladding layer, wherein the refractive indices for silicon and silicon oxide are about 3.47 and 1.45, respectively. In optical systems, e.g. a data communication system using a light with a wavelength equal to about 1310 nm, or a telecommunication system using a light with a wavelength equal to about 1550 nm, a silicon waveguide usually has a height at about 200 to 350 nm, and has a width at about 370 to 470 nm, to ensure good single-mode light transmission. To receive or transmit light signals, light needs to be coupled between a waveguide and an optical fiber. But an optical fiber has a dimension at least 30 times larger than that of a waveguide. For example, a single-mode fiber typically has a diameter of at least 8 micrometers. Due to the extremely different dimensions of the waveguide and the optical fiber, direct coupling would incur tremendous light loss. It is thus essential to meticulously design a waveguide light coupling apparatus for light mode field matching to the fiber dimension. While an outgoing light from a silicon waveguide is usually in transverse magnetic (TE) mode and can be vertically coupled to a fiber using single polarization grating coupler, an incoming light to a silicon waveguide is usually in an unknown and arbitrary polarization state, such that a polarization splitting grating coupler (PSGC) is needed to provide polarization light in either transverse magnetic (TM) or transverse magnetic (TE) polarization mode from the optical fiber to the waveguide.

In one embodiment, a PSGC may be a two-dimensional (2D) grating coupler formed by two single polarization grating couplers nearly perpendicular to each other. Each single polarization grating coupler has elliptical grating lines with the major axis parallel to the fiber azimuth. The total grating area of the PSGC is larger than a core size of the fiber. The PSGC includes scattering elements at the intersection of grating lines on the 2D grating. Each scattering element may have a top surface whose shape is a concave polygon having at least 6 corners and/or at least 8 edges. Different designs of the scattering elements are disclosed to reduce the polarization dependent loss and improve light coupling efficiency from the optical fiber to the 2D grating coupler.

The sizes of the scattering elements on the disclosed grating coupler may be the same or different. In one embodiment, the scattering elements become gradually larger along a first direction from a first convex side of the 2D grating to a first concave side of the 2D grating, the first convex side being opposite to the first concave side; and the scattering elements become gradually larger along a second direction from a second convex side of the 2D grating to a second concave side of the 2D grating, the second convex side being opposite to the second concave side. This helps to improve the coupling efficiency of the grating coupler, because different scattering element sizes cause apodization to the optical coupling, which increases mode field matching of the PSGC to the fiber.

illustrates an exemplary block diagram of a device, in accordance with some embodiments of present disclosure. It is noted that the deviceis merely an example, and is not intended to limit the present disclosure. Accordingly, it is understood that additional functional blocks may be provided in or coupled to the deviceof, and that some other functional blocks may only be briefly described herein.

Referring to, the devicecomprises an electronic die, a light source die, a photonic die, an interposerand a printed circuit board (PCB) substrate. The electronic die, light source dieand the photonic dieare coupled together through input/output interfaces (not shown) on the interposer. In some embodiments, the interposeris fabricated using silicon. In some embodiments, the interposercomprises at least one of the following: interconnecting routing, through silicon via (TSV), and contact pads. In some embodiments, the interposeris to integrate all components including the electronic die, the light source die, and the photonic dietogether. In certain embodiments, each of the dies//are coupled to the interposerusing a flip-chip (C4) interconnection method. In some embodiments, high density solder microbumps are used to couple the dies//to the interposer. Further, the interposeris coupled to the PCB substratethrough wire bondingor through silicon-vias (TSV)using soldering balls. The TSVscan comprise electrically conductive paths that extend vertically through the interposerand provide electrical connectivity between the electronic dieand the PCB. In some embodiments, the PCB substratecan comprises a support structure for the device, and can comprise both insulating and conductive material for isolation devices as well as providing electrical contact for active devices on the photonic dieas well as circuits/devices on the electronic dievia the interposer. Further, the PCB substratecan provide a thermally conductive path to carry away heat generated by devices and circuits in the electronic dieand the light source die.

In some embodiments, the electronic diecomprises circuits (not shown) including amplifiers, control circuit, digital processing circuit, etc., as well as driver circuits for controlling the light sourceor elements in the photonic die. In some embodiments, the light source diecomprises a plurality of components (not shown), such as at least one light emitting elements (e.g., a laser or a light-emitting diode), transmission elements, modulation elements, signal processing elements, switching circuits, amplifier, input/output coupler, and light sensing/detection circuits. In some embodiments, the light source dieis on the photonic die. In some embodiments, the photonic diecomprises an optical fiber arrayattached thereon, an optical interface and a plurality of fiber-to-chip grating couplers. In some embodiments, the plurality of fiber-to-chip grating coupleris configured to couple the light sourceand the optical fiber array. In some embodiments, the optical fiber arraycomprises a plurality of optical fibers and each of them can be a single-mode or a multi-mode optical fiber. In some embodiments, the optical fiber arraycan be epoxied on the photonic die.

In some embodiments, each of the plurality of fiber-to-chip grading couplerenables the coupling of optical signals between the optical fiber arrayand the light source dieor corresponding photodetectors on the photonic die. Each of the plurality of fiber-to-chip grating couplerscomprises a plurality of gratings and a waveguide with designs to improve coupling efficiency between the optical fiber on the corresponding waveguide, which are discussed in details below in various embodiments of the present disclosure.

During operation, optical signals received from a remote server attached on one end of the optical fiber arraycan be coupled through the fiber-to-chip grating couplersattached to the other end of the optical fiber arrayto the corresponding photodetectors on the photonic die. Alternatively, optical signals received from the light source diecan be coupled through the fiber-to-chip grating couplersto the optical fiber arraywhich can be further transmitted to the remote server.

illustrates a top view of an exemplary two-dimensional (2D) grating coupler, in accordance with some embodiments of the present disclosure. As shown in, the 2D grating coupler is formed by two single polarization grating couplers nearly perpendicular to each other. Each single polarization grating coupler has a respective taper region and a shared grating region. The first single polarization grating coupler includes a first taper structureand the shared grating region; and the second single polarization grating coupler includes a second taper structureand the shared grating region.

In one embodiment, the first taper structure, the second taper structureand the shared grating regionare all formed in a planar layer, which may be a semiconductor layer, e.g. a silicon layer on a silicon-on-insulator (SOI) substrate. In one embodiment, the first taper structureis formed in the planar layer connecting a first convex sideof the 2D gratingto a first waveguidein the planar layer; and the second taper structureis formed in the planar layer connecting a second convex sideof the 2D gratingto a second waveguidein the planar layer.

As shown in, each single polarization grating coupler has elliptical grating lines that are concentric elliptical curves. The first single polarization grating coupler includes a first set of concentric elliptical curves; and the second single polarization grating coupler includes a second set of concentric elliptical curvesthat are rotated proximately 90 degrees to form a two-dimensional (2D) grating. The two single polarization grating couplers share the grating regionincluding the 2D grating and an array of scattering elementsarranged in the planar layer at a plurality of intersections of the first set of concentric elliptical curves crossing with the second set of concentric elliptical curves. Any numbers of elliptical curves in each single polarization grating coupler and any numbers of scattering elementson each elliptical curve can be used and are within the scope of the present disclosure.

In a first embodiment, the grating couplerscatters incident light received from the first waveguidein a direction perpendicular to the grating curvesalong the radius direction A, a direction from the first convex sideof the 2D grating to a first concave sideof the 2D grating, the first convex sidebeing opposite to the first concave side. In a second embodiment, the grating couplerscatters incident light received from the second waveguidein a direction perpendicular to the grating curvesalong the radius direction A, a direction from the second convex sideof the 2D grating to a second concave sideof the 2D grating, the second convex sidebeing opposite to the second concave side. In either the first embodiment or the second embodiment, the incident light is scattered out of the 2D grating, which includes periodic gratings formed by the array of scattering elements.

In a third embodiment, the 2D grating couplerscatters incident light received from a fiber having a core sizeattached to the 2D grating. The 2D grating coupleris configured for splitting the incident light received from the fiber on top of the planar layer to a parallel polarization component and an orthogonal polarization component. In this embodiment, the first waveguidecomprises a first output port located substantially at a focal point of the first set of elliptical curves; and the second waveguidecomprises a second output port located substantially at a focal point of the second set of elliptical curves. The 2D grating couplercouples the parallel polarization component to the first output port in the first waveguidevia the first taper structure; and couples the orthogonal polarization component to the second output port in the second waveguidevia the second taper structure. Alternatively, the 2D grating couplercan couple the orthogonal polarization component to the first output port in the first waveguidevia the first taper structure; and couples the parallel polarization component to the second output port in the second waveguidevia the second taper structure.

As shown in, the first taper structurehas a reducing first width from the first convex sideto the first waveguide; and the second taper structurehas a reducing second width from the second convex sideto the second waveguide. In one embodiment, the first taper structureis configured for transmitting a first portion of the incident light from the fiber to the first waveguideto achieve a minimum insertion loss; and the second taper structureis configured for transmitting a second portion of the incident light to the second waveguideto achieve a minimum insertion loss. The first portion of the incident light is substantially a parallel polarization component of the incident light, and the second portion of the incident light is substantially an orthogonal polarization component of the incident light. Each of the parallel polarization component and the orthogonal polarization component comprises a polarized light split from the incident light. The polarized light has either a transverse magnetic (TM) polarization mode or a transverse magnetic (TE) polarization mode.

As shown in, in the first set of concentric elliptical curves and the second set of concentric elliptical curves of the 2D grating, each elliptical curve has an equal spacing relative to an adjacent concentric elliptical curve, where the spacing may be configured as a grating period of the 2D grating.

In one embodiment, each scattering element is a pillar into the planar layer, where the pillar has a top surface whose shape is a concave polygon.illustrates an exemplary scattering elementin a 2D grating coupler, e.g. the 2D grating couplerin, in accordance with some embodiments of the present disclosure. The scattering elementinhas a shape of a concave polygon having 2 reflex interior angles,and 8 edges in total. The concave polygon has reflection symmetry about a linecrossing the 2 reflex interior angles to divide the concave polygon into two equal convex pentagons. As shown in, the concave polygon has 6 corners that do not have reflex interior angles.

As shown in, the concave polygon has 2-fold rotational symmetry; but has no N-fold rotational symmetry, when N is larger than 2. That is, the concave polygon will look exactly the same after a rotation by an angle of 360°/2=180′; but will look exactly the same after a rotation by an angle of 360°/N, when N is larger than 2.

illustrates a top view of an exemplary 2D grating couplerwith an optical fiber, in accordance with some embodiments of the present disclosure. As discussed above, both the optical fiberand the 2D grating couplermay be attached to or included in a photonic die on a substrate. The 2D grating couplerincludes an array of scattering elementson the photonic die for transmitting light between the photonic die and the optical fiber.

In one embodiment, a total area of the array of scattering elementsin the top surface of the planar layer is slightly larger than a core sizeof the optical fiberand is determined based on a diameter of the optical fiber. In one example, when the optical fiberhas a diameter of about 8 to 10 micrometers, the core sizeof the optical fiberis about 15 to 20 micrometers.

In one embodiment, the 2D gratingof the coupleris configured for receiving an incident light from the optical fiberwith an incident angle, as shown in. The incident angleis measured in plane of incidence between an axis of the optical fiberand the Z direction, a direction perpendicular to the planar layer. The plane of incidence is a plane which contains the surface normal of the planar layer and the propagation vector of the incident light. That is, the plane of incidence is the plane formed by the Z direction and the X direction.

Referring to bothand, the lineis along the Y direction, which is a direction in a top surface of the planar layer and perpendicular to the plane of incidence of an incident light from the optical fiber. In one embodiment, each scattering elementhas a first length along the Y direction, and has a second length along the X direction that is in the top surface and perpendicular to the Y direction. In one embodiment, a ratio of the second length to the first length may be determined based on the incident angle. As shown in, the concave polygon is symmetric about a linealong the Y direction and is symmetric about a linealong the X direction. In some embodiments, the incident anglecan be configured in a range of 5-15 degrees according to the structure, geometry, pattern, and material properties of the 2D grating couplerincluding the scattering elements.

Polarized light with its electric field along the plane of incidence is referred to as transverse-magnetic (TM) polarized, while light whose electric field is normal to the plane of incidence is called transverse-electric (TE) polarized.also shows the direction of the electric field of an incident light, when the incident light is TM and TE polarized respectively. As shown in, the electric field of an incident light is along the X direction when the incident light is TM polarized; and is along the Y direction when the incident light is TE polarized.

In one embodiment, the incident angle of the incident light from the fiber is zero. Each scattering element in this embodiment may have a shape of a concave polygon that is in the top surface of the planar layer and has a 4-fold rotational symmetry. That is, the concave polygon will look exactly the same after a rotation by an angle of 360°/4=90°. In this case, the ratio of the second length to the first length is equal to one.

In another embodiment, the incident angle is non-zero; and the ratio of the second length to the first length is larger than one. As such, each scattering element in this embodiment may have a shape of a concave polygon that is in the top surface of the planar layer and does not have a 4-fold rotational symmetry. That is, the concave polygon in this embodiment will not look exactly the same after a rotation by an angle of 360°/4=90°. In other embodiments, the ratio of the second length to the first length becomes larger as the incident angle becomes larger.

illustrates a top view of a portionof a 2D grating coupler, in accordance with some embodiments of the present disclosure. As shown in, the grating portionincludes two scattering elements,, adjacent to each other on a grating curve, which may be one of a set of concentric elliptical curves of a 2D grating region of the grating coupler.

In one embodiment, the scattering elements,are formed in a semiconductor layer. In one embodiment, the scattering elements,comprise a dielectric material such as silicon oxide, while the semiconductor layercomprises a semiconductor material such as silicon.

illustrates a cross-sectional view of the 2D grating coupler portionalong the direction A-A′ in, in accordance with some embodiments of the present disclosure. In the illustrated embodiments, the 2D grating couplerfabricated on a semiconductor substratecomprises a multi-layered structure comprising an insulation layerand a semiconductor layer.

In the illustrated embodiment, the semiconductor substratecomprises silicon. The insulation layercomprises a dielectric material such as silicon oxide, and is fabricated on the semiconductor substrateusing chemical vapor deposition, physical vapor deposition, etc. In some embodiments, the insulation layercan be replaced by other types of dielectric materials, such as Si3N4, SiO2 (e.g., quartz, and glass), Al2O3, and H2O, according to various embodiments of the present disclosure.

In some embodiments, the semiconductor layercomprises silicon and is deposited on the insulation layerusing chemical vapor deposition. In some embodiments, the semiconductor substrate, the insulation layerand the semiconductor layerare formed as a silicon-on-insulator (SOI) substrate.

In some embodiments, the scattering elements,are formed according to a predetermined pattern as shown in. In some embodiments, the scattering elements,are formed as part of a cladding layer comprising silicon oxide. In some embodiments, the cladding layer can comprise other types of dielectric materials according to different applications, including polycrystalline silicon and silicon nitride.

In some embodiments, the 2D grating couplermay further comprise: a bottom reflection layer that is located between the semiconductor substrateand the insulation layerand comprises at least one of the following: Al, Cu, Ni, and a combination; and/or a top reflection layer that is located on the cladding layer and comprises at least one of the following: Al, Cu, Ni and a combination. In some embodiments, the top reflection layer only covers the taper structures of the 2D grating coupler. In some embodiments, the taper structures (not shown) of the 2D grating couplercomprise the same material used in the semiconductor layer. In other embodiments, the taper structures comprise a second material that is different from the material used in the semiconductor layer.

illustrate cross-sectional views of a portion of an exemplary grating couplerat various stages of a fabrication process, in accordance with some embodiments of the present disclosure.is a cross-sectional view of the grating coupler-including a first layerand a second layerdisposed on the first layer, at one of the various stages of fabrication, according to some embodiments of the present disclosure. The first layermay be formed of silicon or another semiconductor material as a substrate. The second layermay be formed of silicon oxide or another oxide material as an insulation layer.

is a cross-sectional view of the grating coupler-including a semiconductor layerformed on the insulation layerat one of the various stages of fabrication, according to some embodiments of the present disclosure. The semiconductor layermay be formed by an epitaxial growth of a semiconductor material, e.g. silicon, on the insulation layer.

is a cross-sectional view of the grating coupler-including a hard mask layerdeposited on the semiconductor layerat one of the various stages of fabrication, according to some embodiments of the present disclosure. The hard mask layeron the semiconductor layermay comprise an organic or inorganic material.

is a cross-sectional view of the grating coupler-including a photoresist layerdeposited on the hard mask layerat one of the various stages of fabrication, according to some embodiments of the present disclosure. The photoresist layeron the hard mask layermay comprise a photoresist material.

is a cross-sectional view of the grating coupler-including patterned portions of the photoresist layer, formed on the hard mask layerat one of the various stages of fabrication, according to some embodiments of the present disclosure. The photoresist layeris patterned according a predetermined pattern, e.g. by removing portions corresponding to the scattering elements shown in, based on waveguide lithography and development.

is a cross-sectional view of the grating coupler-including patterned portions of the hard mask layer, formed at one of the various stages of fabrication, according to some embodiments of the present disclosure. Because the photoresist layerwas patterned to have openings over the hard mask layer, the portions of the hard mask layerthat are exposed by the photoresist layerare removed, e.g., via a wet or dry etch procedure.

is a cross-sectional view of the grating coupler-, where the photoresist layeris removed at one of the various stages of fabrication, according to some embodiments of the present disclosure. For example, the photoresist layermay be removed by a resist stripping.

is a cross-sectional view of the grating coupler-including an array of etched regions,, formed at one of the various stages of fabrication, according to some embodiments of the present disclosure. Because the hard mask layerwas patterned to have openings over the semiconductor layer, the portions of the semiconductor layerthat are exposed by the hard mask layerare removed, e.g., via a wet or dry etch procedure, to form the array of etched regions,.

In some embodiments, surfaces of the etched regions,may be smoothed by: oxidizing the silicon surfaces of the etched regions,; etching the silicon oxide surfaces; and repeating the oxidizing and the etching several times to smooth the surfaces of the etched regions,.

is a cross-sectional view of the grating coupler-, where the hard mask layeris removed at one of the various stages of fabrication, according to some embodiments of the present disclosure. For example, the hard mask layermay be removed by a resist stripping.

is a cross-sectional view of the grating coupler-including a cladding layer, which is formed at one of the various stages of fabrication, according to some embodiments of the present disclosure. The cladding layermay be formed by depositing a dielectric material such as silicon oxide over the semiconductor layerand into the array of etched regions,.

is a cross-sectional view of the grating coupler-, where the top portion of the cladding layeris polished at one of the various stages of fabrication, according to some embodiments of the present disclosure. The top portion of the cladding layermay be polished to form an array of scattering elements,in the array of etched regions,, e.g. based on a chemical-mechanical polishing process.

illustrates a top view of an exemplary 2D grating couplerwith apodization scattering patterns, in accordance with some embodiments of the present disclosure. The 2D grating couplerinis the same as the 2D grating couplerin, except that the 2D grating couplerhas a grating regionincluding 2D grating formed by an array of scattering elementsthat become gradually larger along the Adirection from a first convex sideof the 2D gratingto a first concave sideof the 2D grating, the first convex sidebeing opposite to the first concave side; and become gradually larger along the Adirection from a second convex sideof the 2D gratingto a second concave sideof the 2D grating, the second convex sidebeing opposite to the second concave side.

In one embodiment, there is a same first distance between centers of every two adjacent scattering elementsalong the Adirection; and there is a same second distance between centers of every two adjacent scattering elementsalong the Adirection. The first distance may be equal to the second distance.